This disclosure is related to the field of directional drilling of subsurface wellbores. More specifically, the disclosure is related to optimizing performance of directional drilling using steerable drilling motors.
Wellbores drilled through subsurface formations are known in the art to be drilled along selected geodetic trajectories (“directional drilling”) so as to traverse a path from the surface location of the well to one or more selected subsurface target positions located at predetermined depths and geodetic locations away from the surface location. One technique for directional drilling known in the art is to use “steerable motors” as part of a drilling tool assembly disposed proximate a bottom end of a drill string. A steerable motor is a device which couples within a drill string and is operated to rotate a drill bit coupled to an output end of the motor. The motor may be operated, e.g., by drilling fluid pumped through the drill string by one or more pumps disposed at the surface. Operating components of the motor that generate rotational energy to turn the drill bit are disposed in a housing that has a bend along its length. The angle subtended by the bend may range from a fraction of a degree to several degrees, depending on the particular selected trajectory for any part or all of a directionally drilled wellbore. Steerable motors are operated in one of two modes. In “rotary drilling” mode, the entire drill string, including the steerable motor, is rotated from equipment on a drilling unit (“rig”) at the surface. The equipment may be a kelly/rotary table combination or a top drive. In rotary drilling mode, the direction along which the well trajectory exists (defined by geodetic azimuth and inclination from vertical) is maintained substantially constant, that is, the direction of the well does not change. When it is desired to change the well trajectory in any aspect, the rotation of the drill string is stopped and the steerable motor is oriented so that the bend in the motor housing is directed toward the intended change of direction in the well trajectory. Such operation is known as “slide drilling.”
It is known in the art that slide drilling typically reduces the rate at which the wellbore is drilled (“rate of penetration”—ROP) as contrasted with rotary drilling. Thus, in order to minimize the time of a particular wellbore drilling operation, it may be desirable to minimize the amount of time engaged in slide drilling to drill the well along the selected trajectory. However, minimizing the sliding distance may require higher trajectory change rates, which may be limited by equipment capabilities and can result in increased wellbore tortuosity. Increased wellbore tortuosity may, for example, cause complications during wellbore completion operations. Therefore, the slide drilling—rotatory drilling sequences should be planned such that the overall speed of drilling is balanced with wellbore quality. Further, while the trajectory change effected by slide drilling for any particular configuration of steerable motor and drilling tool combination may be predicted with some degree of accuracy, the actual well trajectory response of any particular steerable motor and drilling tool combination may be affected by factors that may not be precisely known a priori, as non-limiting examples, the mechanical properties and spatial distribution thereof of the various subsurface formations, manufacturing tolerances in the drilling tool assembly and the particular steerable motor, the variability of the actual drilling parameters used (i.e., execution variability, namely the amount of time required to obtain the selected motor orientation during slide drilling may be highly variable and the ability to hold the correct orientation may be highly variable. Beyond that, predictions of directional drilling performance are based on assumptions about drilling parameters that may or may not be correct) and how the particular type of drill bit used interacts with the subsurface formations to drill through them to lengthen the wellbore. Still further, variations in the selected orientation angle of the bend in the motor housing may vary during sliding as a result of, among other factors, changes in reactive torque as the torque loading on the steerable motor changes. Such variations are impracticable to eliminate because of such factors as variability in friction between the wall of the wellbore and the components of the drill string and changes in the rate at which certain formations are drilled by the drill bit, among others.